In System-on-Chip (SoC) technology, reference voltages and reference currents for circuit blocks must be accurate and maintain constant values, and not vary with process-voltage-temperature (PVT) variations. Bipolar junction transistor (BJT) is often applied to generate reference voltages/currents.
The base-emitter (pn junction diode) voltage of BJT is symbolized by VBE, and is depicted in the following Formula:VBE=VGO−[VG(Tr)−VBE(Tr)]·T/Tr−(η−β)VT·ln(T/Tr),  (Formula 1)
where VGO is the extrapolated bandgap voltage of silicon at 0° K., Tr indicates the room temperature (quantified by ° K.), T is the absolute temperature in degrees Kelvin, η is a temperature-independent and process-dependent constant, and its ranging is less than 4 depending on doping level, β is the order of temperature dependence of the collector current of BJT (i.e. IC=ICO·Tβ), and VT is the thermal voltage which is directly proportional to T.
Referring to Formula 1, the VBE is disproportional to absolute temperature T. So, the VBE is a negative temperature coefficient voltage, and comprises a constant component VGO, a first negative temperature coefficient component −[VG(Tr)−VBE(Tr)]T/Tr, and a second negative temperature coefficient component −(η−β)VT ln(T/Tr). The first negative temperature coefficient component, −[VG(Tr)−VBE(Tr)]T/Tr, is porpornal to absolute temperature T. The second negative temperature coefficient component, −(η−β)VT ln(T/Tr), is a non-linear component with absolute temperature T variations. In order to generate constant reference voltages/currents by VBE, the first and the second negative temperature coefficient components in Formula 1, −[VG(Tr)−VBE(Tr)]T/Tr and −(η−β)VT ln(T/Tr), must be compensated by different compensation techniques. Finally, the constant component VGO in Formula 1 would be left and used to provide constant reference voltages/currents for other circuits. The circuits are used to provide constant reference, which is relationship with VGO, voltages/currents are named bandgap reference circuits.
FIG. 1 illustrates a conventional bandgap reference circuit disclosed in Curvature-Compensated BiCMOS transistor Bandgap with 1-V Supply Voltage, IEEE JSSC, 2001. The paper transforms the components of the previously described Formula 1, and decreases operational voltages of circuits utilized therein. Referring to FIG. 1, the currents ICATA, IPATA and INL relate to VBE, [VG(Tr)−VBE(Tr)]T/Tr, and (η−β)VT ln(T/Tr) of Formula 1, respectively. In FIG. 1, ICTAT equals to VEB2/R2, which is {VGO−[VG(Tr)−VBE(Tr)]T/Tr−(η−β)VT ln(T/Tr)}/R2}. ICTAT comprises a constant component, VGO/R2, and negative temperature coefficient components, −[VG(Tr)−VBE(Tr)]T/(TrR2), and −(η−β)VT ln(T/Tr)/R2, wherein the first negative temperature coefficient component, −[VG(Tr)−VBE(Tr)]T/(TrR2), is linearly to absolute temperature T variations, and the second negative temperature coefficient component, −(η−β)VT ln(T/Tr)/R2, is non-linear to absolute temperature T variations. The emitter-base (pn junction diode) voltage of BJT (VEB) also follows a Formula, wherein VEB=VT ln(IC/(Area·JS)). Thus, IPATA, which equals to (VEB2−VEB1)/R1, equals to [VT ln(IC2/(1·JS))−VT ln(IC1/(NJS))]/R1. Because the two p-type MOS transistors, M1 and M2, are of the same channel width to length ratio, so that the two PNP transistors, Q1 and Q2, have equal collector currents (IC1=IC2). Thus, the current IPATA, which equals to [VT ln(IC2/(1·JS))−VT ln(IC1/(NJS))]/R1, equals to VT ln(N)/R1. IPATA is a positive temperature coefficient current, which is linear to absolute temperature T variations and is used in compensating for the first negative temperature coefficient component of current ICTAT. Furthermore, the paper designs the current flowing through another PNP transistor Q3 to be independent from the absolute temperature T. Thus, based on the Formula 1, the current INL, which equals to (VEB2−VEB3)/R4, equals to {[VGO−[VG(Tr)−VBE(Tr)]T/Tr−(η−1)VT ln(T/Tr)]−[VGO−[VG(Tr)−VBE(Tr)]T/Tr−ηVT ln(T/Tr)]}/R4=VT ln(T/Tr)/R4. Herein, INL is a positive temperature coefficient current and is nonlinear to absolute temperature T variations. The paper uses INL to compensate for the second negative temperature coefficient component of the current ICTAT. With elaborately designed resistors R1, R2 and R4, the summation of the currents ICTAT, IPATA and INL is a constant value and is not affected by PVT variations. Thus, the reference voltage Vref generated by the current (ICTAT+IPATA+INL) flowing through the resistor R3 is a constant value which is not affected by PVT variations. The reference voltage Vref is suitable for application in SoC systems.
Referring to FIG. 1, the bandgap reference voltage Vref is based on the current summation (ICTAT+IPATA+INL), and the value of (ICTAT+IPATA+INL) is dependent on the value of the resistors R1, R2 and R4. When a bandgap reference circuit of FIG. 1 is designed, the value of (ICTAT+IPATA+INL) is fixed and can not be adjusted. However, in SoC systems, each of the circuit blocks may require a bandgap reference voltage to fit an exclusive reference voltage curvature. Thus, in conventional techniques, each circuit block of the SoC system must correspond to an exclusive bandgap reference circuit as shown in FIG. 1.